1. Field of the Invention
The present invention relates to an optical transmission system and more particularly to an optical transmission system which is capable of quickly shifting to the regular state from a safe light state in which the shutdown state of the transmission path is detected.
2. Description of the Related Art
In an optical transmission system using wavelength division multiplexing (WDM) signal lights, it is very important to control an optical amplifier in accordance with the number of multiplexed WDM signal lights. In the optical amplifier, the control modes of the automatic gain control and automatic level control are widely utilized.
Since the gain is kept at the constant level in the AGC (Automatic Gain Control) mode, a ratio of input level to output level of an optical amplifier is constant and an output level of an optical amplifier varies following an input level. For example, when the number of multiplexed WDM signal lightss input to an optical amplifier changes, a level of the input signal light changes, but since the gain of the signal light of each wavelength is constant, an output level of the signal light of each wavelength is not influenced.
Since a gain of an optical amplifier is controlled to make constant an output level of an optical amplifier in the ALC mode, the information on the number of multiplexed signals is necessary to control an output level of the signal light of each wavelength of the wavelength-multiplexed signal to the target value. For example, if the number of multiplexed input signal lights of the WDM is varied due to the signal add/drop process, an output level of the signal light of each wavelength also changes, resulting in the possibility of generation of transmission error.
In the optical amplifier used for the optical transmission system in which the number of multiplexed WDM signal lights to be transmitted varies, the control for switching the AGC mode and ALC (Automatic Level Control) mode is implemented.
Since the transmission light is possibly released to the external side of an optical fiber in the optical transmission system due to breakdown of the transmission path or a fault such as opening or the like of an optical connector by erroneous operation of a worker, an output of the transmission light must be kept within the safety criterion when a fault is detected.
There is known an automatic power shutdown (APSD) control in which the WDM light output is suspended only in the section wherein a fiber fault is generated by transmitting, if a fault in the transmission path is detected in the down-stream side of the fault point, the fault information to the up-stream side through the transmission path in the opposite direction in order to suppress an output of the optical amplifiers in the up-stream side. An example of this is shown in Japanese Patent Application JP-A No. 77056/2002.
In this APSD control, an output light intensity is maintained within the safety criterion by suspending output of the WDM light from the up-stream side station and an optical supervisory channel (OSC) control light is output. Since recovery from the fault can be detected by receiving the OSC light in the down-stream side of the fault point, the transmitting state can be returned to the regular state by canceling the APSD control in the up-stream side terminal through the opposite transmission line.
FIG. 4 illustrates flows of controls and signals when a fault occurs at the fault point 7 in the optical transmission path 5A in the optical transmission system in which optical transmission is performed between a terminal 1A and a terminal 1B through the optical transmission paths 5A and 5B and the repeaters 2A to 2C are provided in the course of the transmission paths.
In FIG. 4, the WDM signal light output from an optical switch 13A is amplified with a post-amplifier 10A and is then transmitted to the optical transmission path 5A. The transmitted WDM signal light is amplified with the optical amplifiers 20E, 20C, 20A of the repeaters 2C, 2B, 2A and is then input to a pre-amplifier 11A of the terminal 1B.
Moreover, the OSC light is output from an OSC transmitting unit 121A, in addition to the WDM signal light. Each repeater is also provided the OSC receiving units 222A, 222C, 222E and the OSC transmitting units 221A, 221C, 221E. In the terminal and repeater, the OSC signal is branched to the OSC receiving unit before input to an optical amplifier. These terminals and repeaters receive the supervisory control signal from the up-stream side, combine the OSC signals from the OSC transmitting unit, and then output the combined OSC signals to the down-stream side of the transmission path.
When a fault occurs at the fault point 7 in the optical transmission path 5A, the OSC receiving unit 222A of the repeater 2A in the down-stream side detects shutdown of the OSC signal, while the optical amplifier 20A detects shutdown of the main signal (WDM signal light) with an input light monitor (not illustrated). Accordingly, the control unit 22A of the repeater 2A detects generation of a fault.
The APSD control unit 220A transmits the APSD request signal to the repeater 2B using the OSC signal of the opposed transmission line. The APSD request signal is transmitted to the APSD control unit 220B from the OSC transmitting unit 221B via the OSC receiving unit 222D of the repeater 2B.
The APSD control unit 220B shuts down the optical amplifier 20C on the basis of the APSD request signal. Accordingly, the optical repeater 20C is shutdown and an output of the WDM light is suspended only in the section between the repeater 2B and the repeater 2A where a fault is generated in the optical transmission path 5A.
Here, the OSC light is transmitted from the OSC transmitting unit 221 and recovery of the fault at the fault point 7 can be detected by receiving the OSC light with the OSC receiving unit 222A.
In the case of returning to the regular mode from the shutdown state by the APSD control or the like and switching to the backup line from the working line because of generation of a fault, or in the case of newly constituting a network system, gain setting is necessary for each optical amplifier.
Here, there is also known the technology for setting the gain (gain setting by ASE) of an optical amplifier by utilizing the amplified spontaneous emission (ASE) light even if the signal light is not transmitted to the transmission path when the optical transmission system is constituted and/or when recovered from the defective state. An exmaple of this is shown in Japanese Patent Application JP-A No. 23437/2004).
FIG. 5 and FIG. 6 illustrate the procedures for setting the gain of each optical amplifier through gain setting by ASE when the fault at the fault point 7 is recovered from the shutdown state by the APSD control of FIG. 4.
When the fault at the fault point 7 is recovered in FIG. 5, the OSC light transmitted from the OSC transmitting unit 221C is received with the OSC receiving unit 222A and the control unit 22A of the repeater 2A detects recovery of fault at the fault point 7. The control unit 22A transmits the APSD canceling signal to the repeater 2B using the OSC signal of the opposed transmission line and also transmits the ASE setting request to the up-stream side terminal and the optical amplifier 20A. The optical amplifier 20A receives the ASE setting request and shifts to the ASE setting mode.
The APSD canceling signal is transmitted to the APSD control unit 220B from the OSC transmitting unit 221B through the OSC receiving unit 222D of the repeater 2B. The APSD control unit 220B cancels the shutdown state of the optical amplifier 20C on the basis of the APSD canceling signal. The optical amplifier 20C cancels the shutdown state with the APSD canceling signal and thereafter receives the ASE setting request to shift to the ASE setting mode.
The repeater 2C in the up-stream side also receives the ASE setting request via the OSC signal and the optical amplifier 20E shifts to the ASE setting mode.
When the terminal 1A receive the ASE setting request, the optical switch 13A shuts down the input WDM signal light to the post-amplifier 10A. The terminal amplifier 10A which has shifted to the ASE setting mode by receiving the ASE setting request controls the intensity of the excited light for the gain control in order, to output the ASE light in the intensity which is identical to that of the ASE light when a single light is amplified under the state that the input WDM signal light is shutdown.
The down-stream optical amplifier which has shifted to the ASE setting mode with the ASE light output from the termination amplifier 10A also controls the intensity of the excited light for the gain control in order to output the ASE light in the intensity which is identical to that of the ASE light when only a single light is amplified. When the gain control of the optical amplifier in the ASE setting mode is completed and the gain of the same amplifier is set, each optical amplifier transmits the gain setting end message through the OSC light of the opposed transmission line.
In FIG. 6, when the optical amplifier 20A of the repeater 2A located at the next stage of the fault point 7 shifts to the gain control from the ASE setting mode and sets the gain thereof, the OSC transmitting unit 221B transmits the gain setting end message with the OSC light on the opposed transmission line. When the terminal 1A receives the gain setting end message from the optical amplifier of the repeater 2A located at the next stage of the fault point 7, the optical switch 13A of the terminal 1A switches the input WDM signal light, which has been shutdown, to be transmitted to the post-amplifier 10A. Thereby, the light including the WDM light is transmitted from the terminal 1A.
When the optical switch 13 of the terminal 1A is switched for transmission of light, the optical amplifiers up to the optical amplifier 20A from the post-amplifier have already completed the gain setting by the ASE setting mode. Accordingly, the light including the WDM light is transmitted from the terminal 1A, resetting the regular operating state.
In above description, the intensity of the ASE light in the ASE setting mode is set identically to the intensity of the light when a single light is amplified. However, the ASE light intensity is enough when it can be detected with an input monitor of the optical amplifier located in the next stage and it enables setting of the gain even if the signal light does not exist.
The structure for detection of the shutdown state and recovery from the shutdown state has been described for the external transmission path of the terminal or repeater. Meanwhile, a unit structure is generally employed for each function in the WDM light transmitting apparatus.
For example, the unit including a pre-amplifier for amplifying the WDM optical signal with a receiving unit of the terminal is formed in separation from the unit including a demultiplexing unit for demultiplexing the amplified WDM optical signal, consideration should be taken also for a fault in the fiber connecting the units.
FIG. 7 and FIG. 8 illustrate the procedures for detection and recovery of the shutdown state when a fault is generated in the optical fiber 50A connecting the unit 3A including the pre-amplifier 11A and the unit including the DEMUX unit 31A.
In FIG. 7, a fault in the fiber 50A is detected with a power detector (PD) 32A for monitoring input of the unit 3B. Based on the detection result of PD 32A, the pre-amplifier 11A performs the shutdown operation.
Since the OSC light is not transmitted to the fiber 50A, the fault in the fault point 7 is recovered by setting the pre-amplifier 11A to the safe light state for transmitting light of a level satisfying the safety criterion.
FIG. 8 illustrates the re-setting of the transmission system when a fault at the fault point 70 is recovered. When the fault is recovered, the light transmitted from the pre-amplifier 11A in the safe light state is received with the PD 32A. Accordingly, the control unit 12B detects recover of fault in the fault point 70.
The control unit 12B transmits the ASE setting request to the up-stream side terminals and pre-amplifier 11A using the OSC signal on the opposed transmission line. The pre-amplifier 11A shifts to the ASE setting mode by receiving the ASE setting request.
When the terminal 1A receives the ASE setting request, the optical switch 13A shuts down the input WDM signal light to the post-amplifier 10A. The post-amplifier 10A, having shifted to the ASE setting mode by receiving the input WDM signal light, controls the intensity of the excited light for the gain control in order to output the ASE light in the intensity which is identical to that of the ASE light when a single signal light is amplified under the state that the input WDM signal light is shutdown.
The optical amplifier in the down-stream side which has shifted to the ASE setting mode with the ASE light output from the post-amplifier 10A also controls the intensity of the excited light for gain control to output the ASE light in the intensity which is identical to that of the ASE light when the input WDM signal light is amplified. When the gain control of the optical amplifier in the ASE setting mode is completed and the gain thereof is set, each amplifier transmits the gain setting end message via the OSC light on the opposed transmission line.
The optical transmission systems illustrated in FIG. 4 to FIG. 6 have the problem that a longer time is required to set the gain for all optical amplifiers of the repeaters located in the next stage of the fault point from the post-amplifier of the terminal because the control by the APSD control is different from the gain control for recovery to the regular operating state.
Moreover, such optical transmission systems also have a problem in that the control is complicated because the control by the APSD control must be conducted in separation from the gain control such as the ASE setting or the like.
In the optical transmission system illustrated in FIG. 7 and FIG. 8, it is required to transmit the light in a level satisfying the safety criterion and to detect recovery of the shutdown state with a pre-amplifier because the OSC light is not transmitted to the fiber connecting the units.
Moreover, these transmission systems also have a problem that a longer time is required for total gain setting because the gain control such as the ASE setting or the like must be conducted from the post-amplifier located at the other end of the transmission path for the recovery from the shutdown state.